1. Field of the Invention
The present invention relates to a differential assembly, and more particularly to a pre-loaded clutch pack for a limited slip differential (LSD).
2. Description of the Prior Art
Limited slip differentials are well known in the prior art. Typically, clutch assemblies provide required slip limiting properties in the limited slip differentials. A conventional limited slip differential usually comprises a case rotatable about an axis, housing two bevel side gears coaxial with the case. Each of the side gears is intermeshing with bevel planet gears which are supported in the case on an axis perpendicular the axis of the side gears. Between the differential case and the side gears are interposed the respective clutch assemblies, usually in the form of clutch packs comprising alternately stacked clutch plates and discs, and Belleville springs, to provide the limited slip action. The two clutch packs pilot on side gear hub. The plates are keyed to the case, whereas the discs are splined to the side gear hub. Since these plates are alternately stacked, any rotation of the differential gears causes the frictional surfaces of the plates to turn against each other. The LSD clutch packs are provided with a certain initial pre-load. The pre-load is accomplished by using a biasing means (such as Belleville springs) and plates of various thickness, which cause the Belleville springs to compress when the differential is assemble. The pre-load increases the turning resistance between the clutch plates. The LSD clutch pre-load is currently measured in terms of a torque-to-rotation measurement for the differential case assembly. This is a combination of the Belleville spring force and the frictional characteristics of the plates and discs.
The existing designs of the limited slip differentials do not provide proper control of the clutch pack pre-load, because pre-load can not be controlled individually. Only the torque-to-rotation of the fully assembled differential assembly can be measured. This could allow the differential assembly to be built with differently pre-loaded clutch packs (one tighter than the other), and still have the total torqueto-rotate value within specifications. This results in a larger amount of torque available at one wheel than at the other.
Typically, current differential designs require the selecting from four selective thickness plates to obtain the necessary overall clutch pack thickness to compress the Belleville springs. Then the differential gear is assembled. If the desired torque-to-rotate is not achieved, the differential is disassembled and new plates selected. That makes correct pre-loading of the clutch pack of LSD a very laborious process.
Moreover, any wear which occurs in the differential components, such as spherical radii and side gear thrust faces, spherical washers, pinion mates, cross pin, side gears, discs or plates, etc., results in loss of pre-load.
The existing limited slip differential assemblies are usually provided with a spring means (such as Belleville springs) to urge the side gears away from one another to induce frictional engagement of the clutch assemblies. However, under high torque conditions, side gear separating forces overcome the spring force. This extra force can hamper differentiating, and is detrimental in a truck or light utility vehicle application where heavy loads or trailer tow is involved, even though it may be beneficial in a high performance application.
Therefore, the conventional design renders the existing limited slip differential assemblies quite complex in manufacturing, cumbersome and expensive.